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Community and Q&A

Condensation on Supply Ducts in Hot Attic

satinder | Posted in GBA Pro Help on

I am having a problem with my HVAC unit. It is installed inside a closet which is on the same floor and the closet is conditioned space. During the cooling season, the equipment starts to “sweat”. there is condensation of water on the outside of the supply ducts. The supply ducts go from the closet through the roof into the attic from where they supply these upstairs rooms. The ducts in the attic are wrapped in insulation. The HVAC technician said it is probably because the ducts that are within the closet are not insulated and when the air within the closet touches the cold ducts it condenses and causes sweating. So he replaced the supply ducts leaving the unit with ducts that are now insulated in the inside. This was expensive as he dismantled the ducts and replaced them with insulated ducts. But that has not solved the problem. I am still getting sweating. Anybody know the answer to that? I am at my wits end.
I have another unit which is sitting in the attic for another part of my house and it is doing the same thing. There is water condensing on the outside of the unit where the ductwork starts. Why is this happening?
Can an HVAC expert please help me? Are these not installed properly? Is something missing?

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  1. Jon_R | | #1

    > HVAC unit ... inside ... closet .. conditioned space... equipment starts to “sweat”

    One cause is excessive humidity (in the closet). Have you measured it?

    1. satinder | | #3

      If it is excessive humidity, why is it happening to the unit sitting in the attic? That is exposed to outside hot air.

  2. satinder | | #2

    No I have not. Isn't the AC unit supposed to get rid of excessive moisture inside during summer?

    1. Jon_R | | #8

      To some extent, but to varying degrees, almost all AC systems fail to do this under some circumstances. So measure it.

      AC ducts in an unconditioned attic are a different issue - up there, the only solution is insulation (I'd use CC spray foam).

  3. Expert Member
    BILL WICHERS | | #4

    Condensation occurs whenever the temperature of a surface drops below the dew point of the air around it. This means that you are MORE likely to see condensation on ducts in a hot attic, since you have cold ducts in hot, and usually also moist, outdoor summertime air.

    Insulating your ducts is the way to deal with this. Ducts insulated on the inside can help, but don't always solve the problem. I would insulate on the outside, using a wrap or by "boxing them over" with rigid foam. Something like Dow Thermax, which can be left exposed, can work well for this, and you don't need much -- 3/4" or so should be enough. You can also get a rigid fiberglass product with a white facer that is made for this purpose and is commonly used commercially, but it doesn't give you as nice of a finished appearance as Dow Thermax does for something exposed like a duct run in a closet.


  4. Expert Member
    Akos | | #5

    If you have a decent digital meat thermometer, check the outlet temperature from your unit. Sometimes if the ducting or filter is restrictive, the outlet temperature can drop very low and can cause condensation. If your outlet is in the low 40F range, you need to turn up your blower or improve your ducting to increase air flow.

    The other issue could be that you have a lot of air leaks in your place and these air leaks happen to be in your utility closet. This can definitely be the case with ducts running to the attic. If you can feel hot air drafting into the place from either the floor or the the attic area, you need to do a bit air sealing and plug those leaks.

    Once the air flow and/or air leak issues are sorted, your air handler won't sweat.

    1. satinder | | #7

      Thank you Akos. Having dealt with two houses that I have constructed, I have noticed that the HVAC installers are very careless, they do not do a proper job and yet charge an arm and a leg. The equipment has become so sofisticated, I am afraid the installation is not.

  5. satinder | | #6

    Thank you Bill. I am wondering why it isn't standard practice to wrap the supply duct anyway? Why don't HVAC installers do this at the outset? Would anybody know?

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